Daniil Medvedev came from a set down to beat Dominic Thiem 4-6, 7-6 (7/2), 6-4 and win the ATP Finals title in London on Sunday for the biggest tournament victory of his career.

The Russian fourth seed was second best for large periods of the match at the O2 Arena but found a way back into the contest and eventually proved too strong for the world number three.

Thiem weathered early pressure from Medvedev and secured the first break of the match in the fifth game, when the Russian served a double fault.

Thiem, who beat Rafael Nadal in the group stages and came back from 4-0 down in a deciding tie-break against Novak Djokovic in the semi-final, sealed the set in fortunate fashion when his forehand clipped the net and looped over Medvedev's racket.

Medvedev, 24, who came from one set down against Nadal in Saturday's last-four contest, made a solid start to the second set.

But US Open champion Thiem held to love in the fourth game, looking comfortable as the giant Russian sought a way back into the match.

The third seed earned a break point in the following game but Medvedev saved it with an ace and held to keep his nose in front despite intense pressure.

Thiem had chances to break in Medvedev's next service game but yelled in frustration as he squandered his opportunities.

The Austrian took a tumble onto the blue court at the start of the eighth game and found himself break point down but toughed it out, with Medvedev looking stronger as the set wore on.

The set went to a tie-break. Thiem earned a mini-break on the first point and took a 2-0 lead but the Russian stormed back, winning seven points in a row and levelling the match with an ace.

A ragged service game by Thiem in the third game of the decider left the Austrian facing three break points but he dug deep to hold.

The third seed appeared to be struggling to cope with the punishing pace and finally cracked in the fifth game, when Medvedev produced a volley at the net to break and seize the initiative.

Thiem, who also lost last year's final in London, remained under pressure and had to battle hard to avoid going down another break.